Were those U.S. Military personel officially supposed to be in Ukraine when they were killed? Or were those actually U.S. Military? I believe those people were merely "volunteers", and merely civilians. If it was discovered by Russia/Putin that indeed U.S. Military was sent, entered, and was fighting side by side with the Ukrainian Military, then that's a true declaration of WAR.
Basically in the think tanks in US they came up with a threshold of how much help they can send Ukraine before things escalate out of hands. That includes weapons (eg. type and range of missiles given to Ukraine) to troops they deploy (covert, NCOs, etc.). That includes the number and type of military personnel that are being deployed in Ukraine. They do it as much as it remains hidden and doesn't escalate the situation while still be involved enough to prolong this proxy war.
The news of US troops being killed fighting in Ukraine sometimes come out in the mainstream media too like [1] and [2].
[1] https://edition.cnn.com/2022/04/28/politics/american-killed-ukraine/index.html
[2] https://edition.cnn.com/2022/10/26/europe/joshua-jones-ukraine-russia-transfer-intl/index.html
I believe Sithara007's post about Putin could be the truth. It could be U.S. Military Personnel/Special Forces who are acting under the guidance of the C.I.A. to help the Ukrainian military. Those 'personnel" don't wear the American flag, nor are they considered in an official deployment/mission. If they get caught by the enemy, the U.S. government will absolutely deny everything, and they will not do anything to help the "personnel". It's "Mission Impossible". Hahaha.