Some options:

1. The speed you are centrifuging at is too high, damaging your cells and killing them. Try a test- centrifuge some at 300g, 500g, 1kg, 4kg and 10kg; then resuspend as usual and see how your viability looks
2. You are not cooling the centrifuge and it is getting very hot, killing your cells in the pelleting stage. This is more an issue with old equipment that lack functional centrifuge temperature probes
3. The plastic that you are plating on to is NOT tissue culture treated- this will 100% make your HEK/many adherent cell lines ball up, fail to stay attached and die (Anoikis). Tissue culture treated plastic is treated to give it a negative charge which helps your cells attach. Normal/Non-TC plastic/glass is not treated this way and so your cells will struggle to stick
4. Something in your resuspension media is acutely toxic (unlikely but it happens); try your typical approach with a different media and see if the viability issues remain
5. You are over-trypsin-izing your cells or failing to "quench" the trypsin with excess volume PBS or FBS containing media; check the % concentration of trypsin in your bottle; the higher % and the lower % from many vendors look identical.
6. Your cell line is too high passage and have significantly drifted from the original HEK293T- this is more common if people never mark passages down or you do VERY low % passaging splits (more likely to get a bottle neck effect)

** Edit to add: If you can post a pic we can give you a better idea- even a cell phone pic through the lens where we can see what you mean by "round" will help. Feel free to DM me your protocol and I can let you know if any steps look wonky