four month old miniature schnauzer was 95 percent reliable on potty training at 12 weeks and has now regressed hard in the last two weeks, three accidents a day in spots she has never used before and i cant figure out what changed

Millie is 16 weeks tomorrow, female miniature schnauzer, 8.5lb, weve had her since 8 weeks. potty training out of the gate was almost embarrassingly easy, we ran a tight schedule (out every 45 min the first week, every 90 min by week 3, on cue at the door within 10 days), used a bell she rang consistently by week 5, and by week 12 she was going 4-5 hours between potty breaks with zero accidents for two straight weeks. we were feeling pretty smug about our training regimen honestly.

then around week 14 the wheels came off. she started peeing in the living room without ringing the bell, twice in the same day. we chalked it up to a fluke and tightened the schedule back up. by the end of that week we were up to 2 accidents a day in spots she had never used before (behind the couch, in the guest bathroom which she basically never goes in, in the closet), and this past week we hit 3 accidents a day plus one poop accident which honestly floored me because pooping had been the single most predictable thing on her schedule. no bell before any of them. shes still asking to go out for her scheduled breaks and still using the yard correctly when we take her, so its not that shes forgotten what the yard is for, its that shes not communicating the extra needs and its happening at times we would not have predicted.

what we have tried so far. one, tightening the schedule (back to every 90 min including a break at 2am for one week, no improvement). two, watching her more closely for pre-potty signs we might be missing (nothing obvious, she is not circling or sniffing before the accidents, she just goes). three, changing nothing else in her environment because we know consistency matters (same food, same crate, same yard, same walks). four, ruling out UTI with our vet on tuesday, urine sample was clean, no crystals, no infection, vet said she looks great and to keep at it. five, doubling down on rewards when she does use the yard correctly, which she still does most of the time.

what i cant figure out is whether this is a developmental regression thing that happens at this age (i keep seeing references to a "4 month regression" but the specifics of what that looks like are really vague online), or whether we did something to actively break the training without realizing it, or whether the initial fast pickup was misleading us into thinking she had this locked in earlier than she actually did. shes not showing any other behavioral changes, still eats great, still sleeps through the night in her crate, still runs to the door when we grab her leash. just this one thing has fallen apart.

questions for the people who have been through this. one, is the 4 month potty regression a real phenomenon and if so what did yours look like and how long did it last. two, when a dog stops using the bell they had been reliable on, do you go back to actively teaching the bell or is that a lost cause and you build a different communication pattern. three, at what point should i be pushing my vet for something more than a urine test, like is there a threshold of "no this isnt normal regression, something else is going on" that i should be watching for. four, honestly just any reassurance from someone who has come out the other side of this that were not permanently scarring her by the fact that ive definitely raised my voice at the third accident yesterday. thanks in advance