Why do I get cytosine to guanine/adenine transitions in bisulphite treated sequences?
I quickly aligned your sequences, and as far as I can tell, the answer is that your &quot;consensus&quot; sequence is a poor match for the 3 other sequences:
CLUSTAL format alignment by MAFFT L-INS-i (v7.310)


Consensus_cepa_ gaaagccaaccaccacatccgccatccctcacagtatgccaacgagcagctgaatgactc
MV_A_SP6_izreza aaaaatcaatctccacatccgccatcactcacagtatatttac-agtagataaataa-tt
MV_C_SP6_izreza aaaaatcaatctccacatccgccatcactcacagtatatttac-agtaaataaataa-tt
MV_B_SP6_izreza aaaaatcaatctccacatccgccatcactcacagtatatttac-agtagataaataa-tt
                .***..***.* ************** **********... ** **.*. *.***.* *.

Consensus_cepa_ cgaacaacgcaaagcatgacgcccaaaccgacgaacacccaaccgaaaggccacaagcgc
MV_A_SP6_izreza aaaataacgcaaaacataacgcccaaacagacgtactctcaacctaataacctcgaacgc
MV_C_SP6_izreza aaaataacgcaaaacatgacacccaaacagacatactctcaacctaataacctcgaacgc
MV_B_SP6_izreza aaaatgacgcaaaacataacgcccaaacagacgtactctcaacctaatgaccttgaacgc
                 .**..*******.***.**.******* ***. ** *.***** ** ..** ..*.***

Consensus_cepa_ aacgtgcattccaaaaccccatgaaccaccgaatcctgcaatgcacaccacgctcgacgc
MV_A_SP6_izreza aacttacattcaaaaactcgataattcacgaaattctgcaattcacaccaaatattgcat
MV_C_SP6_izreza aacttacattcaaaaactcgataattcacggaattctgcaattcacaccaaatatcgcat
MV_B_SP6_izreza aacttacattcaaaaactcgatgattcacggaattctgcaattcacaccaaatatcacat
                *** *.***** *****.* **.* .*** .***.******* ******* .. . .*..

Consensus_cepa_ cattcgctacagaccacatcgacacgagcgccaagccatccatcacccgaagccatccac
MV_A_SP6_izreza --ttcgctacgttcttcatcgacacgaaaaccaaaatatccattgccaaaaatcattcag
MV_C_SP6_izreza --ttcgctacgttcttcatcgacacgagaaccaaaatatccattgccagaaatcattcag
MV_B_SP6_izreza --ttcgctacgttcttcatcaacacgaaaaccaaaatatccattaccagaaatcattcag
                  ********.  *. ****.******. .****. .******..** .**..***.** 

Consensus_cepa_ ccactcacaaacataccacgaagcacagcaaatatgaagacaaaccctc
MV_A_SP6_izreza acgctcactgaaataacacaaaacacatcaaa-ataataacaaactttc
MV_C_SP6_izreza acgctcactgaaataacacaaaacacatcaaa-ataataacaaactttc
MV_B_SP6_izreza acgctcactgaaataacacaaaacacatcaaa-ataataacaaactttc
                 *.***** .* *** ***.**.**** **** **.* .******..**

I would suggest trying to come up with a different, more appropriate consensus/reference sequence. This one does not appear to be appropriate.
Granted, I have no idea how you came up with this consensus sequence (it certainly is not a consensus of the other sequences you show), and possibly there is something else going on that I am not understanding.
This does not seem to have anything to do with the bisulfite conversion, as the data also include changes that cannot be attributable to bisulfite, gaps, etc.
Knowing more about the whole workflow would help.
UPDATE
Also, while I'm not familiar with the workflow, could such gaps not be related to methylation on the opposite strand? E.g. CGTA--&gt;TACG.
Is there a particular methylation context that you are looking at? CpGs only?
In some cases there is support for weird variants (e.g. consensus is not an outgroup).
If it's sanger data, it's always a good idea to directly inspect traces.