Algorithm 6 sounds exactly like algorithm 5 in modern/OPL mode because it doesn't calculate more than one operator in the feedback loop. Algorithm 4 is completely wrong too (3 operators in feedback loop). Mark I mode works because it has special case functions to calculate these (2 ops fb, 3 ops fb).
https://github.com/asb2m10/dexed/blob/master/Source/msfa/fm_core.cc#L113
Algorithm 6 sounds exactly like algorithm 5 in modern/OPL mode because it doesn't calculate more than one operator in the feedback loop. Algorithm 4 is completely wrong too (3 operators in feedback loop). Mark I mode works because it has special case functions to calculate these (2 ops fb, 3 ops fb).
Bug report from here: https://www.kvraudio.com/forum/viewtopic.php?p=7847236#p7847236