heal chant is 6.5 seconds, so with very tiny latency you might be losing the difference.
sticking to chants with pulse messages or printouts of when they fade it should be easy to test:

start AF chant (pretty sure that gives a pulse msg) = 6s pulse.
immediately start a resistance chant (gives spell fades msg) = 5s pulse
cancel resistance chant by hitting the spell again before it repulses
check chat log for when resists faded.

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OR.
im maybe a goober and there is a very minor difference with huge repurcussions. maybe all the chants i used on live were 5s chants, and endurance was a 6s chant on live.

I just searched through all the patch notes and cant find any note saying the endurance pulse was changed form 5 to 6 seconds (whether at our patch set or otherwise). but the current herald lists the endurance chants as 6s pulse. yet the old herald (if viewed from web archive) lists it as 5s pulse. so with conflicting "official" information, the obvious answer would be to test it simply from delve before they load up the new endurance chants and screw up any way of testing.

If you could jsut verify endurance regen pulse/duration, even from delve+screenshot is probably what is wrong.

even getting a 6s endurance chant would probably make twisting multiple chants viable here (5s resistance chant and 6s endo would leave you a 1s window after a pulse to retap endo to keep it up).

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I know the experience i had on live, but i no longer have my nostromo or macros that went with it to view the timing. and VN deleted thousands of archived posts including the ones where i had listed all the timing for complex twisting. so its quite possible that a 6s endo chant results in the same times. and that makes more sense because i never remember ending up with weird fractional times for durations like of X.25 etc.

Netchel wrote:heal chant is 6.5 seconds, so with very tiny latency you might be losing the difference.
sticking to chants with pulse messages or printouts of when they fade it should be easy to test:

start AF chant (pretty sure that gives a pulse msg) = 6s pulse.

as i said..tested with both heal and af chant (printouts from af)...for both there was no dmg add even though i triggered dmg add and af at the same moment (well maybe 50ms between)...maybe your other theory is right