Multiselect greyed out when the appropriate tag has a single value

[gaben]

What steps will reproduce the problem?

1. Add a restaurant
2. Choose only one cuisine from the multiselect menu in the preset editor
3. Apply
4. Reopen the editor

What is the expected result?

The multiselect is usable.

What happens instead?

It's greyed out, indicating something unexpected happening in the background (unknown value [not in the preset], wrong separator) but that's not the case.

Please provide any additional information below. Attach a screenshot if possible.

Regression from r18222 as wrote in #21319.

Interestingly if you add two or more known tags, it works.

URL:https://josm.openstreetmap.de/svn/trunk
Repository:UUID: 0c6e7542-c601-0410-84e7-c038aed88b3b
Last:Changed Date: 2021-09-12 03:30:18 +0200 (Sun, 12 Sep 2021)
Build-Date:2021-09-12 01:31:50
Revision:18218
Relative:URL: ^/trunk

Identification: JOSM/1.5 (18218 hu) Windows 10 64-Bit
OS Build number: Windows 10 Pro for Workstations 2009 (19043)
Memory Usage: 765 MB / 7266 MB (359 MB allocated, but free)
Java version: 1.8.0_301-b09, Oracle Corporation, Java HotSpot(TM) 64-Bit Server VM
Look and Feel: com.sun.java.swing.plaf.windows.WindowsLookAndFeel
Screen: \Display0 1920×1200 (scaling 1.00×1.00)
Maximum Screen Size: 1920×1200
Best cursor sizes: 16×16→32×32, 32×32→32×32
System property file.encoding: Cp1250
System property sun.jnu.encoding: Cp1250
Locale info: hu_HU
Numbers with default locale: 1234567890 -> 1234567890
Dataset consistency test: No problems found