That's literally what it says: sometimes when the GBP was validated by phone or text messages, Google will just not allow the phone number to be changed subsequently whatsoever. The edit is just silently rejected within minutes.
When that happens, you have to go through Google Support, which quality has degraded since we started encountering this bug. I genuinely do not know how capable they are of handling the bug when you don't have a track record like my agency does, with documented ticket numbers of seven different clients with the EXACT same problem over the last three years.
This is why I'm very confident that this bug exists. It does not occur on every GBPs that uses these validation methods, but we only see it when that's what was used. It's frequent enough that it should be part of the automatic email that they send out (no human ever reads the initial support request).
BTW, I should also warn you that the solution Google applies is literally to reset validation status and force you to revalidate via postcard.