JabRef version <4.3.1> on
Steps to reproduce:

I am not sure I understand how the bibliography is supposed to look like. Can you paste an example? I can try to modify the stylefile to match your needs. If you still have the problem.
The citation in the above example should look like this:
M.S. van Kester. A Meta-Analysis of Gene Expression Data Identifies a Molecular Signature Characteristic for Tumor-Stage Mycosis Fungoides / M.S. van Kester [et al.] // Journal of Investigative Dermatology. – 2012. – Vol. 132. – № 8. – P. 2050-2059.
There should be an et.al. string according to the style, but it is ignored. It the authors formatter is used only once per citation, then everything is ok. If more than once, then the first formatter is ok, but subsequent are formatted in a weird way. You can see in the first example text marked with red
Check, whether this style-file works for you. Right now, it will only display 1 author for the main author list (after the title) and I defined the [et al.] not as et al string but to be inserted after the authors.
NAME
Belarus
JOURNALS
Belarus Journals
PROPERTIES
Title="Used Literature."
IsSortByPosition="true"
IsNumberEntries="true"
ReferenceParagraphFormat="Text body"
ReferenceHeaderParagraphFormat="Heading 2"
CITATION
BracketBefore="["
BracketAfter="]"
BracketBeforeInList=
BracketAfterInList=.
CitationSeparator=", "
UniquefierSeparator=","
GroupedNumbersSeparator="-"
MinimumGroupingCount="3"
FormatCitations="false"
CitationCharacterFormat="Default"
PageInfoSeparator="; "
LAYOUT
article=format[Authors(FirstFirst,InitialsNoSpace,FullPunc,1,1,EtAl=)]{author}. title / format[Authors(FirstFirst,InitialsNoSpace,FullPunc,1,1,EtAl=)]{author} [et al.]) // begin{journal}journal. - end{journal}begin{year}year. - end{year}begin{number}â„–number. - end{number}begin{pages}P.format[FormatPagesForHTML]{pages}.end{pages}
here the jstyle file.
This issue will be closed in 7 days due to inactivity :zzz: Please provide the requested information if the problem persists.
Most helpful comment
Check, whether this style-file works for you. Right now, it will only display 1 author for the main author list (after the title) and I defined the [et al.] not as et al string but to be inserted after the authors.
NAME
Belarus
JOURNALS
Belarus Journals
PROPERTIES
Title="Used Literature."
IsSortByPosition="true"
IsNumberEntries="true"
ReferenceParagraphFormat="Text body"
ReferenceHeaderParagraphFormat="Heading 2"
CITATION
BracketBefore="["
BracketAfter="]"
BracketBeforeInList=
BracketAfterInList=.
CitationSeparator=", "
UniquefierSeparator=","
GroupedNumbersSeparator="-"
MinimumGroupingCount="3"
FormatCitations="false"
CitationCharacterFormat="Default"
PageInfoSeparator="; "
LAYOUT
article=format[Authors(FirstFirst,InitialsNoSpace,FullPunc,1,1,EtAl=)]{author}. title / format[Authors(FirstFirst,InitialsNoSpace,FullPunc,1,1,EtAl=)]{author} [et al.]) // begin{journal}journal. - end{journal}begin{year}year. - end{year}begin{number}â„–number. - end{number}begin{pages}P.format[FormatPagesForHTML]{pages}.end{pages}