Okay so.
In my experience with the Mega S, the aluminum backed PCB heat bed is flimsy as fuck. That metal thing your build plate clips to? Flexes easier than a wet Pringle, repeated use is causing it to warp and warp QUICKLY.
I tried putting 3mm aluminum sheet IN-BETWEEN the PCB and the glass sheet to get it to stay rigid, that worked for like 60% of the bed, until I realized it wasn't entirely the fault of the bed/glass/aluminum sheet anymore.
I replaced the aluminum plate that the carriage attaches to (the lm8uu bearings+retainers bolt to this sheet, it's black by stock from anycubic), that ... Kinda took care of it as well, but it seemed my warp moved from one end of the bed, to the other.
I said fuck that shit, because there is a chance that the metal rods that the bearings ride along and the retainers that mount those rods to the chassis could or have warped already, is it likely? No, but I'll say this... I've got one flat as fuck bed now as a result.
I ended up replacing the metal rods with linear bearings off of scamazon for 44$, using these mounts (https://cults3d.com/en/3d-model/tool/anycubic-i3-mega-linear-rail-upgrade) that I printed standing up because my damned bed was warped towards the edges and I couldn't get it to lay flat successfully.
Anyway, it's quieter, the prints are more reliable, and I'm very happy with the results.
Do you want a parts list of everything I used?
F it, here's the parts list.
Linear rails [2pcs/Pack] SIMAX3D 3D Printer... https://www.amazon.com/dp/B08ZXPZ839?ref=ppx_pop_mob_ap_share
Y carriage (I had to do some drilling to make it work 100%, mostly used the old one as a template.) [DIYE] Y Carriage Plate Upgrade... https://www.amazon.com/dp/B07PTD7DHW?ref=ppx_pop_mob_ap_share
Rubber spacers for between the carriage and the bed, fuck bed leveling as often as I had to. BCZAMD Heatbed Silicone Leveling... https://www.amazon.com/dp/B07RZKF8MB?ref=ppx_pop_mob_ap_share
Lube all the things (use qtips. Wear gloves, don't fuck with this stuff) DuPont Teflon Non-Stick Dry-Film... https://www.amazon.com/dp/B00BCVXUR6?ref=ppx_pop_mob_ap_share
Longer screws and better (subjectively) thumb nuts Befenybay 4PCS Upgraded Hand... https://www.amazon.com/dp/B07RFMSNGQ?ref=ppx_pop_mob_ap_share
Just preprint errthang if you do this. Also print a backup (or two) of the linear rail mounts, I broke one because I grabbed the wrong M3 bolt length and had to slapdash print one on my franken-kossle that has some accuracy issues...
You'll have to get creative with the y-endstop, if you do this. I'll snap a photo of how I mounted mine. Picture long ass m3 bolts with some nuts holding it in place parallel to the linear rail with the endstop lever bent a smidge. It works but is a bit g h e t t o
EDIT ADD If I could make one last recommendation as well, get a silicone based hot bed, Something similar to this, I can't remember the exact bed dimensions, but this will also, help prevent warping. https://www.amazon.com/temperature-Ender-3s-Flexible-235x235mm-Silicone/dp/B07GJDL7XB/ref=sr_1_12?keywords=3d%2Bprinter%2Bsilicone%2Bpad&qid=1637677438&qsid=145-0161930-5878959&sr=8-12&sres=B07GR55L1Y%2CB07FB1TZ25%2CB07FTD4SSC%2CB0814YRGGK%2CB07DWPYQDS%2CB07ZFPKHN5%2CB07VCT22DJ%2CB07GDHX2Q6%2CB07GJDL7XB%2CB08BNR7ZS9%2CB07FQ7FQHG%2CB01ERAE0FY%2CB076H1RBJW%2CB07MR7Q986%2CB089D9SL8R%2CB01N9HGKR1&th=1
I truly hope whomever finds this in the future asking about bed warping, potato chipping, prints that wont stick, flex bed, whatever, this helps...