BLS-2017 has been updated to 29.05.04, so I would first do that update and look at this again.
Without having your league information or any other data handy, it's difficult to be able to pinpoint something. But, I'll take a few stabs at it.
I'm functioning only on intuition here, but since you say the handicap is higher than it should be...is it remotely possible that the gender for said leadoff bowler changed somehow (inadvertently)? I'd look at that, and preview a listing of his scores history to see if anything in that record shows something amiss, his status as a team member vs. substitute, or extra bowler (roster greater than playing strength).
If you're using a PRO version of BLS-2017, I would also do the following: With the scores entry screen open for this team, click the "Options" button at the top of the window, and check the box for "Position for each game" (lower right of all boxes available, but only available in a PRO version), then click "Update Form". An additional column will appear to the right of each bowler game, and if they're green, they should be good and should contain a range of the team strength. If you see a number that's for an odd reason higher than the playing strength (such as a 5 for a 4-bowler team) that may be grayed out, you can change that to a correct number to possibly fix things.
Again, these are only remote possibilities and without seeing your league, it's difficult to know exactly what's going on, which is why a question like yours gets responses containing more questions. You gave a version number and a Windows version, both are good. It would always help, though, to at least say something like "this bowler had previous season pinfall of X, over Y games, giving him a Z average. Our handicap rule is A (could be a team or bowler rule of B percentage off a C basis score) where we'd expect handicap to be D, but instead we are seeing BLS give him or the team a handicap of E." And if you are using a pin-drop rule, mention that also in case the rules aren't properly set up for that...or anything else that may be in play. Even something odd occurring last week could affect something, including possible makeup games not yet posted, requiring you to go back one or more weeks.
I hope I can at least give you a little insight and get you back to a working league file. The ultimate correction may be far simpler than this, of course.
Always check, download, and install the most recent update of your software to ensure that issues may have been corrected.