tomboy does not find obvious search words

Behaviour:

What it should do:

1. Click on "Search All Notes" to open the main Tomboy interface.
2. Enter a search key: e.g. "hello"
3. Click on the note that matches.
4. The search key is highlighted and the search bar at the bottom shows number of matches.

What it does with some search keys:

1. Click on "Search All Notes" to open the main Tomboy interface.
2. Enter a search key: e.g. "TFSA"
3. Click on the note that matches.
4. The search key is not found in the note.

I'll attach screen shots below.

I have a lot of notes (550), but my Nokia N900 running Conboy deals with them fine.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: tomboy 1.15.9-0ubuntu1
ProcVersionSignature: Ubuntu 4.15.0-45.48-generic 4.15.18
Uname: Linux 4.15.0-45-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.5
Architecture: amd64
CurrentDesktop: XFCE
Date: Sun Feb 10 12:07:31 2019
InstallationDate: Installed on 2019-02-09 (1 days ago)
InstallationMedia: Xubuntu 18.04.1 LTS "Bionic Beaver" - Release amd64 (20180725)
SourcePackage: tomboy
UpgradeStatus: No upgrade log present (probably fresh install)