There are two factors at play here:
1. The other party forgot to log you. This does happen sometimes. An email to them asking them to examine their "ALL.TXT" file often gets you the QSO in their log. N1IG shows how the log typically looks.
2. Different stations have different standards on what "complete" means. Most of us adopt (even for HF) the VHF standard that in FT8 terms works like this: Once both stations have received and sent an "R" message, they are entitled to log it. The trailing 73 is optional.
A few stations will log based on a single exchange. Many more insist on seeing the trailing 73 or RR73 before they will log. But most follow #2.
So this exchange:
CQ W1HAM FN42
.... W1HAM K1HAM EN35
K1HAM W1HAM -03
.... W1HAM K1HAM R-05
K1HAM W1HAM RR73
or even K1HAM W1HAM RRR
. . . may be regarded as complete. W1HAM should log this QSO. However, if K1HAM does not hear the RR73, then K1HAM may not log the QSO.
To be sure of it, W1HAM should look for the trailing 73. Alternatively, if the W1HAM K1HAM R-05 message repeats, then W1HAM needs to resend the RR73 message because K1HAM certainly did not log it in that case.
However, even when K1HAM gives W1HAM nothing more than the R-xx message, in the vast majority of cases, both parties did log the QSO and by ancient VHF standards (again, now widely adopted on HF) this is a valid QSO, if a little ambiguous for W1HAM (but not for K1HAM in that case).