Reporting!!
So I have many devices and try data saver on each of them.
In PC I use Chrome, Edge, and Firefox. All of them seems ok. Loading become faster (~30s to ~5s). Resolution becomes lower but that's expected.
In my android smartphone I use Samsung Internet. Everything seems ok. Similar with PC.
In my iPad, the data saver broke the manga reader in every browser that I used (Safari, chrome, and firefox).
Only the first page shows up and I cannot change page (even when the page indicator shows I already in different page).
When I change to long strip, some pages will be partially cut (usually after around 3/4 of the page).
My guess is that in iPad, all browsers use webkit (safari engine). Therefore, it will broke the manga reader no matter what browser you choose because all of them use the same engine.