I Fred-swapped Bulwark out for Greater Shield Mastery. It should have giving me PRR+5 & DS+2 versus ISM (I only have 13 US -- not enough for PRR from Bulwark). It only gave DS+2. Tried reglogging. Still no PRR change. Zeroed all DP from US (removed Legendary Shield Mastery) and lost 15 PRR (as it should), but still did not gain the extra 5 PRR from GSM.

Now it gets weirder. I removed my buckler. No change. It should have PRR-10 or at least PRR-5 if GSM PRR was missing. Nope, the PRR was unchanged. So, I Fred-swapped GSM out, leaving SM+ISM. Still no PRR change swapping my buckler in and out.

Next, I tried swapping in a Large Shield I had handy. Now it gave the appropriate PRR+20 (10 Large 10 ISM) and swapping the Buckler started giving the correct PRR+5 when swapped in and loosing it when swapped out.

So, it looks like either GSM is not working and/or there are bugs swapping Shield Mastery feats with Fred that relogging does not fix. You might be able to fix them by swapping in a different weight shield, though.

At this point, I've punted using GSM in favor of something else. At 5 PRR 2 DS, it's not really worth a feat slot and certainly not worth burning shards testing when I could use something that works instead.