Some servers advertise some of the mods by treating the Steam ID of the mod as a 64bit number and setting the highest 32 bits to ones, creating a huge number.
I found this by looking at the A2S_RULES packets but then confirmed it in the Arma 3 launcher. Some mods that I don't have subscribed are detected as such and the launcher suggests subscribing to them (correct behavior).
Others (the ones bugged) can't be found and the launcher displays the "choose action" red box.
Example of such a server:
TeTeT USAF and FA-18 BETA/South Asia Server 188.8.131.52 2902
Mods on the server: [(ID, name), ...]
[(1325500, ''), (18446744071737540240, '@saws'), (463939057, '@ace3'), (1849403283, '@toh_south_asia'), (1865403034, '@toh_core'), (1552807725, '@f18_beta'), (18446744071811956195, '@usaf_fighters'), (18446744071811960366, '@usaf_util'), (18446744071811945151, '@usaf_main'), (450814997, '@cba_a3')]
ID Without the highest 32 bits:
>>> 18446744071737540240 & 0xffffffff 2322955920
Correct workshop URL for @saws, using the "fixed" ID:
>>> 2322955920 | 0xffffffff00000000 18446744071737540240
You can see the mods with huge numbers are the ones that are not detected correctly by the launcher: