Bold text in special headings

Hi guys,

For our designs needs we have used bold text in the special headings text as shown in this image https://ibb.co/RCKg1BS. For 7-8 months now i have been getting this error message: https://ibb.co/DDWywyp the solution i was given was to use the special character converter plugin and replace all the bold codes with these characters: ###lt###b###gt###ΜΟΝΟΔΡΟΜΟΣ###lt###/b###gt### but if that is the only solution i would have to go to thousands of pages and change it as i have many customers with the same special heading design. Is there any other solution that is more reliable and also for what reason am i getting this error message. Also i have the same issue when i try to put links in the sub heading below the special heading, it does not accept normal html code. Please help if there is any solution as i will have to put it in thousands of pages if not. Please note i do not want to use the normal text block as it does not have the design options that the special heading does.

Thanks in advance.