This is a young team, with a few veterans picked up at the trade deadline.
The young pitchers to me are showing the effects of a long season, not loss of velocity but loss of command as they start getting "tired arm" syndrome.
As SD showed, 98 MPH FBs that aren't located well get hit in today's game, hitters without bat speed have been selected off ML rosters, so velocity is relatively less important (because who can throw 100+ on a regular basis?).
Eflin, Pivetta and Velasquez throw 94+ and get hit when they can't locate their secondary pitches or put their FB where they want to.
It's pretty usual for young pitchers to struggle with command, as Schilling? mentioned, in AA pitchers have to stay out of the middle of the plate, in AAA they have to stay within 8" on either side of the plate, in the Show, 4" on either side of the plate. If you miss the plate, you fall behind in the count, if you miss the "safe zones" ML hitters will pound you.