With Google I found a couple of sources saying that, while the Form 8606 should be included in the IRS transcript, the IRS sometimes fails to pick up the 8606, or picks up only part of the information from the form. So it might be that the IRS system has some bugs in handling this form.
Another 8606 thread - IRS transcript shows all zeros
(from 2018)
"The failure is at the IRS, not with the major tax programs."
Reconstructing Lost IRA Basis To Help Avoid Double Taxation
(by Jeffrey Levine, CPA, 2021)
"In situations where a client with basis in their Traditional IRA cannot find a tax return with Form 8606 attached, one option would be to ask the IRS for a copy of the Return Transcript for previous years. Theoretically, this transcript should indicate any amounts reported on Form 8606. Unfortunately, though, it’s not uncommon to find Return Transcripts lacking such information, even when Form 8606 was included with the return." [emphasis added]
