After drying out an Ocean Monument & doing a thorough search of all rooms, it was apparent it spawned without a treasure room. There were three rooms with sponges (about 95 total), a separate room with a prismarine column in one of the wings, an arrangement of Sea Lanterns and columns plus an arch in the other wing, but no treasure room, no gold blocks. The rear of the central area where the Treasure room normally is found was completely empty floor to ceiling.
Screenshot shows "empty" rear of monument, taken from the farthest rear wall, looking to the front, torches mark structure centerline.
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After finding & drying out another (and mostly complete) OM and comparing its internal structures with the one reported on above it seems that at some point in its generation cycle, the "empty" monument's generation was interrupted. The prismarine column & arch present in one wing (see above) is an incomplete version of structures found in corresponding areas of the second OM, and a number of rooms besides the Treasure Room were never generated, leaving the large empty space shown in the screenshot. If spawning of the Elder Guardians follows the OM structural generation (as seems logical) then their spawning was also interrupted.
In the second OM, no sponges were found, and only two Elder Guardians were present, that in the Penthouse, and in the wing containing the separate room with the prismarine column. After killing those two I was not again afflicted with Mining Fatigue, and only regular Guardians were found in the remaining wing.
Additionally, after reviewing info on the OM content, it seems there were no Elder Guardians spawned either, though plenty of regular Guardians were. The Penthouse room was empty, both wings had only regular Guardians, and at no time did anything try to afflict me with Mining Fatigue.