Hi Bernard,
Thanks for contacting Syncfusion support.
We checked the reported problem but we are unable to reproduce it at our end as the headerText characters are shown properly with the bootstrap4 theme.
Please find the below sample prepared based on this for your reference,
Please share us the below details to validate further on this,
1) Share complete grid rendering code.
2) Let us know Syncfusion package version and EJ2 Grid component theme file referenced in the application.
3) Please let us know, if you have used any custom CSS in your sample? If so, please share it
4) If possible share us a simple sample to replicate the problem or try reproducing it in the above provided sample.
Regards,
Rajapandiyan S