Thank you for your question and the photo, Anna.  We can't provide a definitive diagnosis with just this information, but I suspect your tree has the following nutritional problem:  https://pnwhandbooks.org/plantdisease/host-disease/cherry-prunus-spp-june-drop

As you can see, there are many causes, including lack of fertilization (which you can impact); the weather (which we can't); and pollination (although Lapins are self-pollinating).  This article has information about fertilization, which may be the primary cause.  https://extension.oregonstate.edu/gardening/berries-fruit/fertilizing-fruit-trees   I hope this is helpful.  Good luck!

Welcome!  Do you have grass around the tree?  Most grass fertilizer is mostly nitrogen, which stimulates foliage.  A more balanced fertilizer has phosphorus and potassium as well.  That may be why your tree's leaves look good, and this drop happens.

Good!  Just a heads up.  The DIY soil kits are not as reliable as labs, whose testing doesn't cost much more.  And, for a few dollars more, they can recommend specific nutrients to add for the "target" plant species.  https://extension.oregonstate.edu/catalog/pub/em-8677-analytical-laboratories-serving-oregon

Welcome!