I dont know what the cause of the issue Ronski discovered is.
But what I will say is this.
A while back (several years now), FreeBSD developers decided to modify PF the firewall imported from OpenBSD to better scale to extra cpu cores, at the time it was considered a great thing to increase PPS performance caps, but then because this was done, no one wanted to merge in future PF updates from OpenBSD as because of this patch, the code was no too different. This standoff situation lasted for multiple years with more and more outstanding bugs in PF/ALTQ left unresolved, then another discussion happened, and it was decided to still not update PF *sigh* but at least the developers are now starting to maintain PF and do FreeBSD bug fixing for it. OpenBSD developers maintain that the version of PF in FreeBSD is now considered an outdated buggy mess.
I have been using FreeBSD since the 4.x days, a long long time. I migrated my servers to PF not long after it was ported over, as I considered it a large step forward from ipfw. However in all the years I have been using PF it has become apparent there is a lot of bugs, many of these are either very minor or can be worked around, some cannot be worked around and are just there, ipv6 has a fair amount of buggy behaviour in FreeBSD PF, one of which PF will not pass on fragmented packets, and when running cloudflare's fragment tester tool, this will be evident if you are behind a PF firewall.
For as long as opnsense pfsense are based on FreeBSD they effectively have to adopt the buggy PF, opnsense has a couple of hardenedbsd developers now on their team, who have been porting over openbsd security features, I would absolutely love for opnsense to move to OpenBSD, and then they would have the proper fixed modern PF, and I think that move alone would drag over a big part of pfsense's userbase to them.
As for bugs that only affect pfsense but not opnsense, a possible explanation is that pfsense do patch parts of the kernel with custom networking code, this kernel source code is no longer publically available (a reason why many devs jumped ship to opnsense), so it is possible these patches have introduced even more bugs than base FreeBSD.
It is on my to do list to migrate to opnsense at home, I no longer use pfsense in datacentres, they all migrated to opnsense a long time ago.
There is also little niggly things that are not bugs but have been implented in opnsense but not pfsense.
So e.g. in opnsense I can block outbound dns requests not directed to my LAN ipv6 dns server, and actively reroute them to the LAN dns server, the same way that I do on ipv4, so basically I enforce all of my LAN to use my firewall DNS server on both stacks. On pfsense this is only possible on ipv4, because even though its in pf command line, I think one one of the lead dev's massively aganst NAT66, NAT46 etc. and even though this is not technically NAT, it is seen as a NAT type feature so the rdr feature is ipv4 only on pfsense.