Simple explanation - pio calling it because don't want to run set over flush(for example when we hit 8).
More advance that it has more equity that's why we are calling it and not calling without FD blocker. (but as you can see EV isn't close)

u block 1 out vs his FDs
u have more bluffcatching river potential ( not sure if u can call some river )
but 88 without diamond unblock more bluffs but probably u are folding river always

no idea about strong SH ranges but
i feel 88 cant bluffcacth most of rivesr and if u will be mostly folding u prefer block outs vs his flushdraws ( check is 8x8d can continue on river d some %)

i think this will change if u have a hand that will mostly call rivers , because u prefer let him bluff , but in this case 88 should be call turn + fold river

You need to look at a few different spots to try to understand why this is happening. While posters above are probably correct there may be other reasons it happens in different spots. It depends on range construction.

Start by going back to the Bet 60 node and look at villains strategy. What combos does 8d affect? Now go to the river and look at the EV of this combo on different runouts.

Easiest way to do this is to use hotkeys that are built into piosolver for changing runouts (rank up = Ctrl+Up, etc). This can also be done for previous streets.

Another useful tool is hotness tool (ctrl+H).