Have you tried switching back to bupe after using fentanyl before? I went through a period of doing fent analogues whilst I was on bupe, as it was able to break through the blockade effect of 8mg. It was period of about 6 or 8 weeks, eventually every time I switched back, it took longer and longer before I could switch without getting some level of PWD. It reached the stage where it wasn't mild symptoms, it was major, crippling PWD that lasted a good 24 hours for the acute symptoms and 3 days for the after effects.At the end, even waiting 48 hours didn't help, in fact that one was the worst. It was also the last time I did it, I switched to methadone about 10 days after stopping both the fent and bupe.
Whilst I can't help you with the methadone to bupe switch, I think one of the big factors in getting PWD seems to be a combination of duration of use, dose and number of times you have switched from another opioid back to bupe. If you've had PWD symptoms in the past, chances are you'll get them again and worse than the previous time.
How long before you took the methadone did you last take fent? I think you should be OK if it was 48 hours since you last took the fent and at least 24 hours since you took the methadone, since it was only 1 dose and a relatively small dose at that. Take 2mg first, wait an hour or 2 then take more if you don't get bad symptoms
Have a quick read here:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5976217/
It's a study on transferring people to bupe from a range of methadone doses. None of the patients on low dose methadone, which is under 30mg, experienced PWD. They were taking methadone everyday, maining their blood levels are much higher than if they only took a single dose like you did. I'd be more concerned with the fentanyl TBH. Whilst it has a short half life, it takes much longer to be eliminated from the body with chronic use.