I'll leave the Castra change below but I've just finished testing an even simpler Igrys fix for this which I have posted on the Discussion page there:
https://mods.factorio.com/mod/Igrys/discussion/69cae4ec35b14be82d52fc36
Hopefully that one will get implemented fairly quickly and an update will work rather than requiring manually changing a mod.
ORIGINAL MESSAGE:
If you're happy to modify your version of the mod, you can change line 361 of control.lua from:
if allPrereqs and allSciencePacks then
to
-- Exclude techs that Igrys makes mutually exclusive: researching
-- either one causes the other to be un-researched on the enemy force.
local igrys_mutex = (research.name == "igrys-magic-speed" or
research.name == "igrys-magic-reach")
if allPrereqs and allSciencePacks and not igrys_mutex then
I've also modified info.json and changelog.txt in mine to change the version to 0.5.5 but that's purely a cosmetic change.
For testing: I have loaded a save with this update where we had the researches bouncing back and forth and it then switched to a different recipe when the next one finished. I haven't done any longer term testing yet but the scope of the change is small enough that I wouldn't expect it to cause any other problems