Indexing a file does not work when it's uploaded to a directory that is synced with Nextcloud desktop app

It seems that the sync process of the desktop app changes the behaviour of fulltextsearch. Instead of indexing the file it tries to delete the (nonexisting) index (see below).

• file uploaded to non-synced directory:

│ Action: waiting
│ Provider: Files                Account: myuser
│ Document: 1394759
│ Info: image/jpeg
│ Title: Testfile.jpg
│ Content size: 1292
└──
┌─ Results ────
│ Result:      5/5
│ Index: files:1394759
│ Status: ok
│ Message: {"_index":"nextcloud","_id":"files:1394759","_version":1,"result":"created","_shards":{"total":2,"successful":1,"failed":0},"_seq_no":39742,"_primary_term":1}

• same file uploaded to a directory which is synced by nextcloud desktop app (in my case Linux version 3.9.3):

│ Action: waiting
│ Provider: Files                Account: myuser
│ Document:
│ Info:
│ Title:
Memory: 63 MB
┌─ Indexing  ────
│ Action: waiting
│ Provider: Files                Account: myuser
│ Document: 1394759
│ Info: image/jpeg
│ Title: Testfile.jpg
│ Content size: 1292
└──
┌─ Results ────
│ Result:      6/6
│ Index: files:1394767
│ Status: success
│ Message: index deleted  

Reproduce the issue by uploading the same file to a normal directory and to one synced by the Nextcloud desktop app on Linux, then compare the indexing output. Investigate the full-text indexing path responsible for the differing document IDs and deletion result; done means the synced upload is indexed successfully rather than deleting a nonexistent index.