Why do not all the coinbase texts have the block heigh in it? The coinbase text of #511329 starts with a, #511330 starts with b, #511331 starts with c... (and so on until z), but #511426 for example starts with F, the next one with m, the next one with "... regarding https://coin.dance/blocks

Why do some of them follow the rules, but others not?
All coinbase transactions do contain the block height, and have done so since block 227,835.  It's required by BIP 34.  Since all other miners enforce this rule, a block which broke it would get orphaned and would not become part of the block chain.

You're simply not looking at the right thing.   The height is encoded by a single byte giving the number of bytes to follow (currently 0x03) and then the height as a little-endian binary number.  Blockchain.info displays the coinbase transaction scriptsig both in hex and as characters, but not all bytes correspond to a printable ASCII character, so it displays a � character for such bytes (there seems to not be a one-to-one correspondence between bytes and characters, so it may be trying to parse it as Unicode or something).

Decimal 511329 is hex 0x07cd61, so its coinbase starts with the bytes 0x03 0x61 0xcd 0x07 (remember, little endian).  Now 0x03 is unprintable and 0x61 is a, so you see an a in that text.  Decimal 511330 is hex 0x08cd62, so you see 0x03 0x62 0xcd 0x07, and 0x62 is b.

But decimal 511426 is hex 0x07cdc2, so the coinbase starts 0x03 0xc2 0xcd 0x07.  None of these correspond to any printable character.  The transaction goes on as 03c2cd07ce46... and 0x46 is ASCII F.  This happens to be the first printable character in the coinbase transaction, but it isn't part of the block height at all; just part of whatever other data the miner felt like including.  You've ignored the bytes which were not printable, but you can't do that.

Likewise the coinbase for 511247 starts out 03c3cd07fabe6d6d...  Again, the block height is included exactly as it should be, since decimal 511427 is hex 0x07cdc3.  It so happens that the first printable character is 0x6d, ASCII m.  But again  that has nothing to do with the block height.