From your comments it sounds like I should try using less protection above the print and under the platen, perhaps just 1 board instead of two, and perhaps shorten the time. When I mounted inkjet prints earlier in the year (quite successfully), I had to use lower temp but using less protection above those prints showed that the heat pattern was not uniform across the print surface. Adding the extra board (so in total 2 x 4 ply) seemed to make it quite even heat, which I assumed would be a good thing for FB prints, and which is how I had always done it before.
I wish there were not so many variables at play here. It would be so much easier to nail down the issue... I suppose I should try higher temp and shorter time, or, as some suggested, placing the print "upside down", that is print below the tissue below the mount board.
If anyone else reading this sees the picture and feels "a-ha" becaue you had seen this issue, please let me know. I am facing a longish mounting session this week, 18 prints to do, and I really would love to get this solved for now, and I suppose for the benefit of the community.