Well, I figured out the issue: it might have been just an unfortunate coincidence that threw me off the debugging path.
The missing part from the report above is that on the 70.x subnet, the wifi AP is a ubiquity, which relays DHCP requests to pfsense (it sits comfortably on the 70.2 ip, 70.1 is the OPT1 if in pfsense). 70.3+ and until 70.100 are all reserved for static mappings, 70.101+ are the DHCP pool.
At some point I realized that when a DHCPACK came through, it was not completely random. By inspecting the phone's logcat I started to notice a pattern in which the failures to receive the DHCPOFFERS coincided with the phone starting the conversation over the 2.4ghz channel, and then being moved over to the 5ghz channel by the band steering protocols on the ubiquity. Hilarity ensued, since for over a year I had no problem with the same hardware and a unified SSID for both frequencies.
What probably threw me off was the fact that the ubiquity received a firmware upgrade (which likely introduced this issue) right around the same time I manually upgraded pfsense >:(
Solution was: separate the two wifi networks into their own dedicated SSIDs and disable band steering. Once that was done the DHCP protocol did not have any issues completing any handshake, over 2.4ghz or 5ghz.
I'm leaving this here for posterity, might be useful to someone one day.
Cheers