Bold/Italic does not work for some font styles.

This might be a framework limitation in WPF: https://www.telerik.com/forums/some-fonts-are-not-able-to-bold-italic

Sample fonts list:

1. Tw Cen MT Condensed Extra Bold (bold/italic)
   
2. Tw Cen MT Condensed (bold/italic)
   
3. Sitka Text Semibold (italic)
   
4. Sitka Subheading Semibold (italic)
   
5. Sitka Small Semibold (italic)
   
6. Sitka Heading Semibold (italic)
   
7. Sitka Display Semibold (italic)
   
8. Sitka Banner Semibold (italic)
   
9. Segoe UI Variable Text Semibold (italic)
   
10. Segoe UI Variable Text Light (bold/italic)
    
11. Segoe UI Variable Display Light (bold/italic)
    
12. Segoe UI Semilight (bold/italic)
    
13. Segoe UI Semibold (bold/italic)
    
14. Segoe UI Light (bold/italic)
    
15. Segoe UI Black (italic)
    
16. Script MT Bold (bold/italic)
    
17. Rockwell Condensed (bold/italic)
    
18. Rockwell Extra Bold (italic)
    
19. Rage Italic (bold/italic)
    
20. OCR A Extended (bold/italic)
    
21. Nirmala UI Semilight (bold/italic)
    
22. Microsoft YaHei UI Light (bold/italic)
    
23. Microsoft YaHei Light (bold/italic)
    
24. Microsoft JhengHei UI Light (bold/italic)
    
25. Microsoft JhengHei Light (bold/italic)
    
26. Malgun Gothic Semilight (bold/italic)
    
27. Leelawadee UI Semilight (bold/italic)
    
28. Gloucester MT Extra Condensed (bold/italic)
    
29. Gill Sans Ultra Bold Condensed (bold/italic)
    
30. Gill Sans Ultra Bold (bold/italic)
    
31. Gill Sans MT Ext Condensed Bold (bold/italic)
    
32. Gill Sans MT Condensed (bold/italic)
    
33. Franklin Gothic Medium Cond (bold/italic)
    
34. Franklin Gothic Heavy (bold/italic)
    
35. Franklin Gothic Medium (bold/italic)
    
36. Franklin Gothic Demi (bold/italic)
    
37. Franklin Gothic Demi Cond (bold/italic)
    
38. Franklin Gothic Book (bold/italic)
    
39. Dubai Medium (bold/italic)
    
40. Dubai Light (bold/italic)
    
41. Corbel Light (bold/italic)
    
42. Copperplate Gothic Light (bold/italic)
    
43. Copperplate Gothic Bold (bold/italic)
    
44. Cooper Black (bold/italic)
    
45. Cascadia Mono SemiLight (bold/italic)
    
46. Cascadia Mono SemiBold (italic)
    
47. Cascadia Mono Light (bold/italic)
    
48. Cascadia Code SemiLight (bold/italic)
    
49. Cascadia Mono ExtraLight (bold/italic)
    
50. Cascadia Code SemiBold (italic)
    
51. Cascadia Code Light (bold/italic)
    
52. Cascadia Code ExtraLight (bold/italic)
    
53. Candara Light (bold/italic)
    
54. Calibri Light (bold/italic)
    
55. Britannic Bold (bold/italic)
    
56. Bodoni MT Condensed (bold/italic)
    
57. Bodoni MT Black (italic)
    
58. Bernard MT Condensed (bold/italic)
    
59. Bahnschrift SemiLight Condensed (bold/italic)
    
60. Bahnschrift SemiLight SemiConde (bold/italic)
    
61. Bahnschrift SemiLight (bold/italic)
    
62. Bahnschrift SemiBold (bold/italic)
    
63. Bahnschrift SemiBold SemiConden (bold/italic)
    
64. Bahnschrift SemiCondensed (bold/italic)
    
65. Bahnschrift SemiBold Condensed (bold/italic)
    
66. Bahnschrift Light Condensed (bold/italic)
    
67. Bahnschrift Light  (bold/italic)
    
68. Bahnschrift Condensed (bold/italic)
    
69. Arial Rounded MT Bold (bold/italic)
    
70. Arial Narrow (bold/italic)
    
71. Arial Black (italic)