Hello Dresden.
I had Constantine installed but disabled (I'm on steam, disabled with the launcher) and the same for your Alexander mod.
Just in case, I removed Constantine, Alexander mod and DeI to resubscribe to only Dei in order to get a clean version.
And I still had no empire maintenance bonus with the nervii with the drainage and canals buildings. I didn't have the time to see it that was also the case for all the others celtics factions.
Btw, I also had this issue with the Kaledonoi, so I can assume there might be also this issue with the britannic factions.
I can look at the others factions after work if you want though