nice to see this update.one of several things may be going on here.
1. timing.
re gelcaps + mag not working, i've been advised (for good reason) to take some mag at the same time as d3 and to take another portion separately. your post info was unclear re timing specifics. maybe careful timing of mag, around your routine d3 intake, could have mitigated your issues.
2. if d3/mag intakes were correctly timed/coordinated, then consider magnesium dosage.
if the intake via gelcaps was 10,000 IU/d and the gel form was well absorbed, depending on your diet and other personal details, 300 supplemental mag glycinate may not be adequate, with or without proper attention to timing.
some mag math:
7-10 mg / kg body weight of daily mag intake has been recommended in the literature - and at that, outside the context of supplemental d3 intake.
in the context of high dose d3, it may be wise to assume you need *at least* 10 mg / kg body weight of daily magnesium from all sources. this may exceed DRI for your sex and age.
for every added activity that forms a drain on mag resources (such as: stress from any source whether emotional or physical ranging from exercise to injury to surgery; regular use of antacids, specifically PPIs; regular consumption of caffeine and/or alcohol), increased mag intake is in order.
it is always best to maximize food sources before topping up with a supplement. if you don't know the contribution your diet is making (or not) to daily intake, it's worth making the effort to understand and optimize that part of your routine.
3. d3 composition.
strangely enough, it could be because the d3 powder caps are not as good, that that particular d3/mag combo works better to address your side effects. i would need more product info to have any certainty about that. in the meantime:
if the powder capsule intake was also 10,000 IU and 300 mg mag glyicinate worked, it's possible the powder cap form of d3 was less efficiently absorbed, meaning that 300 mg mag glycinate was adequate to the task (with or without considering timing).
sadly if this is the case it may mean you are excreting the excess d3, with unintended downstream impacts.
ps i went back and checked on an earlier mag info link posted above, and it was broken. now repaired.
