corrupt letters in libreoffice

After I leave libreoffice calc open for a few hours, sometimes a single character will display corrupted.

I also got a corruption in the 'y' of the confirmation dialog when I tried to paste in a cell that already contained data.

Closing and reopening libreoffice seems to fix the corruption when it is a letter (I don't know about the confirmation dialog).

Version: 6.1.2.1
Build ID: 1:6.1.2-0ubuntu1.1
CPU threads: 8; OS: Linux 4.18; UI render: default; VCL: gtk3_kde5;
Locale: en-CA (en_CA.UTF-8); Calc: group threaded

ProblemType: Bug
DistroRelease: Ubuntu 18.10
Package: libreoffice (not installed)
ProcVersionSignature: Ubuntu 4.18.0-12.13-generic 4.18.17
Uname: Linux 4.18.0-12-generic x86_64
ApportVersion: 2.20.10-0ubuntu13.1
Architecture: amd64
CurrentDesktop: LXQt
Date: Mon Dec 10 01:23:58 2018
InstallationDate: Installed on 2018-11-15 (25 days ago)
InstallationMedia: Lubuntu 18.10 "Cosmic Cuttlefish" - Release amd64 (20181017.2)
SourcePackage: libreoffice
UpgradeStatus: No upgrade log present (probably fresh install)