Hello and welcome to BW forums
It looks to me like your LO's A time is probably not long enough before naps and then perhaps too long just before bed time, the last A time could be too long due to having had a short nap rather than a long nap and ending up over tired at bed time.
The guidance A time at 6 months is more like 3hrs and you'd be aiming for two long naps of 1.5 - 2hrs each. The 45 min naps you are seeing may be due to being UT (under tired) so can't transition from one sleep cycle to the next, a series of UT naps can then lead to a one off long nap because of accumulated tiredness. Being unable to transition between naps can also be because LO doesn't know how and may need help to transition or some sleep training for naps rather than just night bed time.
To me it looks like the PUPD was working but the routine wasn't quite right.
I'd suggest increasing the A time in increments, not all in one go, so I'd go straight to 2hr 30 on all A times (including before BT, not 3hrs), then after a day or 2 increase to 2hr 45 and leave it there for a few days, see how naps go. After say 3 days you may need to increase to 3 hrs.
At the same time you can use W2S (wake to sleep) to help LO transition between sleep cycles, here's a link, look at naps option 1:
https://babywhispererforums.com/index.php?topic=223809.0If you feel you can't do W2S for every nap then choose one nap per day but make it the same nap each day if at all possible because habits build per nap.
Use W2S in cycles doing it for 3-4 days then hold off for one day to see what happens, if LO wakes then try to resettle and the next day start another cycle of 3-4 days W2S again holding off on day 4 or 5 to see what happens.
As you increase the A times I would suggest for now not to increase the last A time beyond 2hr 45. Waking after BT suggest OT (over tired) soI'd keep the last A shorter to avoid this. See how it goes, you can always increase back to the 3hrs if you feel it isn't working out.
I hope this helps. Let us know how you get on.