Incomplete search display after entering 5 characters

At the beginning I would like to praise the plugin.
The client I maintain the site for reported a small bug (a big one for them)
When the search is done up to the 4th character it shows all 9 items with that SKU number, but adding the 5th character only shows 3 and if it should show all 9.
Here’s an example: all nine items have a same SKU of first 5 numbers 76.839. they differ only by the last number or letter 76.839.1, 76.839.2, 76.839.3, 76.839.A …. When I search for 7683 it shows all 9 but when I search for 76839 it only shows 3.

The site uses the Woodmart theme and so far there have been no problems. I checked all options, regenerated, changed options, turned off plugins and added some custom codes, I turned on debugging… but it didn’t help.
The only interesting thing is that three of the items it shows are new and the other 6 are from before. The same is the case with the other products items (SKU 76.895.)

Thanks in advance!

The page I need help with: [log in to see the link]


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