Hi Niko,
The current behavior of the TreeGrid Column Filtering for string edit will search for content starts with. We have also logged a feature request regarding this. A support incident has been created under your account to track the status of this requirement. Please log on to our support website to check for further updates.
https://www.syncfusion.com/account/login?ReturnUrl=/support/directtrac/incidents
Please let us know if you require further assistance on this.
Regards,
Mahalakshmi K.