Nothing nefarious, CW Skimmer just busted your call. It is not unusual during a contest with lots of signals and qrm, and stations with similar calls. You can download the skimmer spots and check very easily. For example:
DL1AXX,DL,EU,7012.3,40m,N4UM,K,NA,CQ,14,2022-02-20 03:51:52,34,CW
RL3A,UA,EU,7012.3,40m,N4UU,K,NA,CQ,27,2022-02-20 03:51:53,34,CW
KC0VKN,K,NA,7012.3,40m,N4UU,K,NA,CQ,25,2022-02-20 03:51:54,34,CW
LZ7AA,LZ,EU,7012.3,40m,N4UU,K,NA,CQ,28,2022-02-20 03:51:54,33,CW
EA5WU,EA,EU,7012.3,40m,N4UU,K,NA,CQ,39,2022-02-20 03:51:54,33,CW
K5TR,K,NA,7012.3,40m,N4UU,K,NA,CQ,34,2022-02-20 03:51:54,33,CW
K6FOD,K,NA,7012.3,40m,N4UU,K,NA,CQ,13,2022-02-20 03:51:55,35,CW
N2QT,K,NA,7012.3,40m,N4UU,K,NA,CQ,39,2022-02-20 03:51:55,33,CW
K3PA-1,K,NA,7012.3,40m,N4UU,K,NA,CQ,26,2022-02-20 03:51:55,33,CW
WF7U,K,NA,7012.3,40m,N4UU,K,NA,CQ,12,2022-02-20 03:51:55,34,CW
W8WTS,K,NA,7012.3,40m,N4UU,K,NA,CQ,36,2022-02-20 03:51:56,34,CW
HB9JCB,HB,EU,7012.3,40m,N4UU,K,NA,CQ,29,2022-02-20 03:51:56,33,CW
K9IMM,K,NA,7012.3,40m,N4UU,K,NA,CQ,17,2022-02-20 03:51:56,33,CW
HA1VHF,HA,EU,7012.3,40m,N4UU,K,NA,CQ,31,2022-02-20 03:51:57,34,CW
One skimmer copied N4UM, but clearly it was actually N4UU on 40m. Maybe there was QRM or key clicks from another station that messed up the second "U" in UU.
Tor
N4OGW