No, it's not a bug. UE scans always only the first 9 KB (UE v11.20a) or 64 KB (UE v14.00b) of a file to detect it's format. And that's okay because there are users which edit really large files with several MBs or even GBs.
I rarely use UTF-8 encoding. Most of the files I daily edit are normal ASCII files. If UltraEdit would start to check always the whole file for binary chars, UTF-8 multi-byte chars, UTF-16 chars, ASCII escaped Unicode chars, etc. the load time of a file would increase dramatically and we all don't want that.
So I have a question to you: What type of files you encode in UTF-8? HTML or PHP or XML files?
Yes! Why do you not specify the UTF-8 encoding at top of the file as required by the HTML standard?
No, other files! Why do you not use the UTF-8 BOM?
For UTF-8 handling see
Using UTF-8 with UltraEdit, especially the chapter
My suggestions for the configuration for UTF-8 webpage writers in my post and the linked pages there.
By the way: If your files are HTML files why do you not use HTML entities for the German umlauts and the 'ß'? In the HTML toolbar of UltraEdit there is a symbol with the tooltip "HTML Text2HTML" which will convert characters to known HTML entities.
And I personally have following macro associated to a hotkey which does the same as "HTML Text2HTML" with a limited character set.
IfExtIs "html"
Else
IfExtIs "htm"
Else
ExitMacro
EndIf
EndIf
InsertMode
ColumnModeOff
HexOff
IfSel
Find MatchCase "ä"
Replace All SelectText "ä"
Find MatchCase "ö"
Replace All SelectText "ö"
Find MatchCase "ü"
Replace All SelectText "ü"
Find MatchCase "Ä"
Replace All SelectText "Ä"
Find MatchCase "Ö"
Replace All SelectText "Ö"
Find MatchCase "Ü"
Replace All SelectText "Ü"
Find MatchCase "ß"
Replace All SelectText "ß"
Else
Find MatchCase "ä"
Replace All "ä"
Find MatchCase "ö"
Replace All "ö"
Find MatchCase "ü"
Replace All "ü"
Find MatchCase "Ä"
Replace All "Ä"
Find MatchCase "Ö"
Replace All "Ö"
Find MatchCase "Ü"
Replace All "Ü"
Find MatchCase "ß"
Replace All "ß"
EndIf
And additionally I have macros like that with hotkey ä ö ü Ä Ö Ü ß to automatically insert the umlauts and 'ß' in the correct form based on the file extension:
IfExtIs "html"
"ß"
ExitMacro
EndIf
IfExtIs "htm"
"ß"
ExitMacro
EndIf
IfExtIs "asm"
"ss"
ExitMacro
EndIf
IfExtIs "c"
"ss"
ExitMacro
EndIf
IfExtIs "h"
"ss"
ExitMacro
EndIf
IfExtIs "inc"
"ss"
ExitMacro
EndIf
"ß"