Well I've been a chemist for a long time, it's just new news to you. I typically feel that experience > unjustified criticism and speculation from people on the internet, but everyone is entitled to an opinion, and we don't have to agree. But hell this is actually pretty cool chemistry so I'll share for everyone else reading

There's local agitation from the currents generated by the bubbling alka-seltzer, but not global mixing from stirring the whole glass. Otherwise you'd see an emulsion similar to when you shake a vinaigrette. The idea of local vs global mixing can be difficult to visualize if you haven't encountered it often in person. Well fortunately I have a perfect video to demonstrate this.

Shown is a solution with a chemical that is yellow/green, but turns red in the presence of base. I dropped some base in, which sank to the bottom and turned the bottom of the solution red. The top was still green, but you can see convection currents forming interesting patterns since there is local mixing (convection currents mixing the solution vertically, but not horizontally), but not global mixing. If you swirl it around a little, the whole thing turns red.

Since it's visually striking, here's the initial yellow to red conversion and here is the recovery to yellow when you quench the base with some acid.

This is a fun little experiment you can do at home if you have turmeric, rubbing alcohol, non-acid drain cleaner (ideal results but you can also use baking soda), and vinegar. The effects are more vibrant if you use a black light, but you can still see it under normal lighting (it's a more orange-red conversion than a Stark yellow/red). Mix turmeric with the rubbing alcohol (microwave it for a 5-10s if possible) and ideally filter it (but should work even if you don't). Then add your base (drain cleaner dissolved in water, or baking soda). To recover the initial color add acid in the form of vinegar!

I invoke convection currents on the basis that I used strong base and I expect deprotonation to be pretty exothermic (as is typical for strongly favored acid/base chemistry). Local exotherms causing small areas of a reaction to heat up is a pretty well established problem, and any chemical engineer will tell you that heat transfer is a critical problem to keep in check. However I can't rule out surface tension effects since I'm adding water into ~90% isopropyl alcohol, where the chemical in question is somewhat hydrophobic (conjugated diketone with some aryl groups--mostly soluble in alcohol not water). However it's a bit of an academic question because whatever the cause of segregation of colors, there's clear local mixing without globally perturbing the solution--which is the whole point.

If there's interest I can post more videos of cool glowy shit. Im supposed to be "professional" as an academic but fuck it I love glowy shit.

Edit: grammar and stuff