XnView Software

This is a correct behaviour:
In Browser, select an uncompressed TIFF file (24 or 48 BPP).
Tools -> Metadata -> Edit IPTC/XMP... (^I)
[Options] Tab -> Mode: XMP, update or create IPTC-IIM
[Caption] Tab -> Insert "000111222" in "Caption"
Press [Write] button
In "Info pane" / "IPTC-IIM" you can see "000111222"
In "Info pane" / "XMP" you can see "000111222"
In "Info pane" / "ExifTool" you can see "000111222" in both XMP and IPTC section

WARNING: sometimes, when you re-open "Edit IPTC/XMP..." (^I), previously inserted caption text disappear and the "Caption" space is _empty_.
BUT previous inserted text will be shown in full screen "Info".
In my tests, this occur only with TIFF files (I use mainly TIFF) and with "Mode: XMP, update or create IPTC-IIM".

Hint: when this occur, in "Info pane" / "ExifTool" tab you see the Caption previously inserted (XMP section) _BUT_ "Info pane" / "XMP" tab is a completely _blank_ pane.
At this point, opening Tools -> Metadata -> Edit IPTC/XMP... (^I) result in blank "Caption" pane.
If you insert ANY new text in "Caption" and hit "Write", you LOSE previous inserted text because you not see it.

Extra behaviour: long lines is not wrapped and is truncated.

You can test with multiple text lines and numbers.

XnView MP 0.97.1
Win 7x64
Win 10x64

do you have a way to reproduce it?

None. The problem is erratic, but frequent.
I discover this: in "Info pane" / "XMP" you can view XMP Rating and Label, but *not* Description. In this case, opening "Edit IPTC/XMP... (^I)" result in an empty "Caption" field. If you insert any text, previously inserted text is overwritten and lost.
This occur when you set "[Options] Tab -> Mode: XMP, update or create IPTC-IIM".

Addendum:
With Version 0.93.1 64bits (Mar 6 2019):

- IPTC / XMP data loss problem exist (bug IS present)
- Long text lines wrap is OK (bug is NOT present)

Hello,

cicciobello wrote: Tue Oct 20, 2020 2:48 pm XMP_empty_tab.tif

With 'Mode: XMP, update or create IPTC-IIM', XMP is loaded not IPTC, so it's normal that Caption is empty

cicciobello wrote: Tue Oct 20, 2020 2:48 pm XMP_long_line_unwrapped.tif

What's the problem with this file?