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The main issue for reverse popping out is you've grinded the sycnro sleeve and gear down, the only proper fix is to change both pieces, mine did the same thing 5 to 6 tries before it would stop popping out under load, went in to it and found it all ground down.
Wore out gear, notice the teeth are fairly chewed up.
Wore out syncro slider.
New gear, notice the nice sharp teeth.
New slider with sharp teeth also.
New gear with new blocker ring (also know as just the syncro)
And old gear with old blocker ring and this blocker ring still had lots of life left in it too.