Trying to sum elements in a column based on comparing criteria between sheets

I have two sheets: Backlog and Sprint. The backlog items are numbered 1, 2, 3, 4 and so on, and the sprint items are numbered 1.1, 1.2, 1.3, 1.4 and so on.

In the backlog I need to sum the hours spent from column J3:J in the sprint sheet for the items with the same initial ID (1, 2, 3, 4...).

I'm trying to do this but I get 0, even though the sum of column J with values 1.X in col A is 4,5.

=SUMIF(A3;"="&INDEX(SPLIT('Sprint 1'!$A$3:$A;".");1); 'Sprint 1'!$J$3:$J)

The IDs are strings, if that matters. They need to be so because...reasons

Here's a generalized example of how it can be done:

=sumif(map(A:A;lambda(z;index(split(z;".");1)));D1;B:B)