The Textus Receptus is not one fixed text

Which of the two Textus Receptus, which have different wordings, is true?

Revelation 16:5 (King James Version)

And I heard the angel of the waters say, Thou art righteous, O Lord, which art, and wast, and shalt be, because thou hast judged thus.

Revelation 16:5 (New International Version)

Then I heard the angel in charge of the waters say: “You are just in these judgments, O Holy One, you who are and who were;

5 και ηκουσα του αγγελου των υδατων λεγοντος δικαιος κυριε ει ο ων και ο ην και ο οσιος οτι ταυτα εκρινας 

Stephen’s 1550 Textus Receptus: With Morphology (Bellingham, WA: Logos Bible Software, 2002), Re 16:5.

5 και ηκουσα του αγγελου των υδατων λεγοντος δικαιος κυριε ει ο ων και ο ην και ο εσομενος οτι ταυτα εκρινας

Scrivener’s Textus Receptus (1894): With Morphology (Bellingham, WA: Logos Bible Software, 2002), Re 16:5.

This actually gets at what "the Textus Receptus" even means, since as we noted earlier, it isn't one fixed text. We have found a clean example of that: Scrivener's 1894 TR (which underlies the KJV, since Scrivener reconstructed the Greek text the KJV translators were actually working from) follows Beza's ὁ ἐσόμενος, while Stephanus's 1550 edition — an earlier and in some ways more conservative TR edition — has ὁ ὅσιος.

Our assessment: ὁ ὅσιος is almost certainly the authentic reading

Here's the reasoning, weighing what we went through:

Manuscript evidence heavily favors ὅσιος. Of roughly 200 extant Greek manuscripts containing Revelation 16:5, ὅσιος is the overwhelming majority reading — including the earliest and most geographically diverse witnesses. ἐσόμενος appears in no Greek manuscript's main text at all; the only Greek manuscript containing it has it as a marginal note from the late 1600s, almost certainly copied from Beza's own printed edition after the fact rather than independently attesting to it.

Beza's stated source is unverifiable. He claimed to have restored ἐσόμενος from "an old and reliable manuscript" he had access to — but no such manuscript has ever surfaced, and no other scholar since has been able to identify or confirm it. In textual criticism, an unverifiable appeal to a lost manuscript, made by an editor who also had clear theological motive (fitting John's triadic "was/is/coming" pattern), is treated as weak evidence, not strong evidence. Scholars who take Beza at his word are trusting his word alone against 200 manuscripts.

The corruption-direction argument favors ὅσιος as original. Textual critics generally prefer the reading that best explains how the other reading could have arisen as a scribal error. Here, ἐσόμενος → ὅσιος (accidentally shortening a long, unusual word to a shorter, more expected one that also completes the "was/is/will be" pattern in a smoother way) is a bigger stretch mechanically than most accidental corruptions look like. Meanwhile ὅσιος → ἐσόμενος essentially doesn't happen by scribal accident at all — it would have to be a deliberate, reasoned edit, which is exactly consistent with what Beza describes himself doing.

Beza's supporting evidence is suggestive, not conclusive. The Job 15:14 LXX and Clement of Alexandria parallels show ἐσόμενος-style constructions describing God existed in the broader Greek theological vocabulary of the period — but that only shows the phrase was conceivable, not that John actually wrote it here. It's evidence for plausibility, not attestation.

What this means practically

• If you hold to VPP or a hard TR-primacy position, you're in the position of needing to defend Scrivener/Beza's ἐσόμενος specifically — which, notice, actually means defending one 16th-century editor's judgment call against the wider "Received Text" manuscript tradition itself, not defending "the manuscripts" as a block. That's a genuinely awkward spot for a position that grounds its authority in preserved, widely-used manuscripts rather than individual editorial conjecture.
• If you follow ordinary text-critical method (external manuscript weight + internal transcriptional probability), ὅσιος — Stephanus's reading, and also the reading in the modern critical text (UBS/NA) and the Byzantine Majority Text — is the far better-attested and more defensible answer.

So between your two TR editions: Stephanus 1550 (ὅσιος) more likely preserves what John actually wrote; Scrivener's 1894/Beza's ἐσόμενος (and hence the KJV's "and shalt be") is very likely a 16th-century editorial emendation, made in good faith and with real linguistic reasoning behind it, but not attested in the surviving Greek manuscript record.