The original question was about the Word Completion feature of LO Writer and more specifically the treatment of letters and vowel signs (in Punjabi/Gurmukhi) by this feature. I have no idea why the answers so far have spiralled off into a discussion about double-byte encoding. @ROSt52 you almost certainly do not have a supporting Punjabi/Gurmukhi font installed. Even my phone shows the characters fine. Here are the characters for 'Sikhi' and 'Harman' respectively:
$ unicode ਸਿੱਖੀ
U+0A38 GURMUKHI LETTER SA
UTF-8: e0 a8 b8 UTF-16BE: 0a38 Decimal: ਸ
ਸ
Category: Lo (Letter, Other)
Bidi: L (Left-to-Right)
U+0A3F GURMUKHI VOWEL SIGN I
UTF-8: e0 a8 bf UTF-16BE: 0a3f Decimal: ਿ
ਿ
Category: Mc (Mark, Spacing Combining)
Bidi: L (Left-to-Right)
U+0A71 GURMUKHI ADDAK
UTF-8: e0 a9 b1 UTF-16BE: 0a71 Decimal: ੱ
Category: Mn (Mark, Non-Spacing)
Bidi: NSM (Non-Spacing Mark)
U+0A16 GURMUKHI LETTER KHA
UTF-8: e0 a8 96 UTF-16BE: 0a16 Decimal: ਖ
ਖ
Category: Lo (Letter, Other)
Bidi: L (Left-to-Right)
U+0A40 GURMUKHI VOWEL SIGN II
UTF-8: e0 a9 80 UTF-16BE: 0a40 Decimal: ੀ
ੀ
Category: Mc (Mark, Spacing Combining)
Bidi: L (Left-to-Right)
$ unicode ਹਰਮਨ
U+0A39 GURMUKHI LETTER HA
UTF-8: e0 a8 b9 UTF-16BE: 0a39 Decimal: ਹ
ਹ
Category: Lo (Letter, Other)
Bidi: L (Left-to-Right)
U+0A30 GURMUKHI LETTER RA
UTF-8: e0 a8 b0 UTF-16BE: 0a30 Decimal: ਰ
ਰ
Category: Lo (Letter, Other)
Bidi: L (Left-to-Right)
U+0A2E GURMUKHI LETTER MA
UTF-8: e0 a8 ae UTF-16BE: 0a2e Decimal: ਮ
ਮ
Category: Lo (Letter, Other)
Bidi: L (Left-to-Right)
U+0A28 GURMUKHI LETTER NA
UTF-8: e0 a8 a8 UTF-16BE: 0a28 Decimal: ਨ
ਨ
Category: Lo (Letter, Other)
Bidi: L (Left-to-Right)
As you can see 'Sikhi' has two letters and two vowel signs (and does not auto-complete) while 'Harman' has four letters (and does auto-complete). The auto-completion of words can be adjusted under Tools > AutoCorrect Options… > Word Completion but I imagine the original poster knows this. The problem is more likely to be whether a combining vowel sign is treated as a 'letter' by the LO Writer feature. I suspect not, but we need an expert to provide greater information.