Diminishing return brackets for skills are 0-55, 56-85, and 86+, which is the same as the secondary stats (with the exception of Primary, which was recently changed).
However, the diminishing returns factors (the reduction in scaling rate at each bracket tier) for skills differ from those of the secondary stats (i.e. Primary, Sidearm, Auxiliary, Robot, Def/Resis).
The scaling factor listed on the Wiki is the base factor and only applies to the first tier (0-55).
For your convenience, I'll make a table of the tiers:
Tier 1 (0-55): Base (as stated on the Wiki)
Tier 2 (56-85): Base*1.1
Tier 3 (86+): Base*1.35
The base scaling rate for Static Charge is 1% per every 5 tech. At tier 2, it diminishes to ever 5.5 tech. At tier 3, it diminishes to every 6.75. This explains the odd scaling behavior that you observed.
As far as I know, all skills should have diminishing returns. I haven't checked each one to make sure, though, so if you think Static Smash, Static Grenade, Assimilation don't then, yes, that may be a bug. But now that you know how diminishing returns for skills work, you should be able to check it yourself. Hopefully, this was helpful.