Good thought, just remembered I had something similar one time when working at a festival. There it was because of wireless microphones that somehow interacted with the phone network.
Maybe some special equipment of the bases use special protocols that interfere with other devices that are not programmed for it.
Yes I only have 4G with my FP3. Maybe Brittany has some newer (or older) cell-towers that LiniageOS (or the FP3) could not handle.
Could be, maybe some frequency band is still not handled well by LineageOS but other phone OSs do, and the military uses those.
I want to go back, this time with my radio analyzing tools just to find out more, but would not be a wise move with today's tensions in Europe... (or in general near military bases)
I think also in the UK where The Guardian (or was it some other paper), found and published the use if IMSI-Catchers on protests?
I am aware of that, but Brest (or better said Ile Longue) is the base where the french nuclear ballistic missile submarines are stationed (https://en.wikipedia.org/wiki/%C3%8Ele_Longue).
In this article you can see the different buildings/bunkers, I went to all of them (as near as you were allowed) was very interesting (https://thebulletin.org/premium/2023-07/nuclear-notebook-french-nuclear-weapons-2023/).
Could be something in this direction, but the when it happened I was some times out in the nature, no building or antenna near me. As far as I know some military equipment can interfere with civilian technology, as they do not care much about it when a war is going on.