This is normal behavior, and not a bug.
consider that there is no text wrapping in the display:
a list of Vlans configured as:
switchport trunk allowed vlan 1, 3, 5, 7, 9, 11, 13, 15, 17, 19-21, 101, 123, 125, 127, 129, 131, 150, 1024, 1666, 1890, 1990
would show as something like:
$127, 129, 131, 150, 1024, 166, 1890, 1990$
Therefore configuration may be entered as:
switchport trunk allowed vlan 1, 3, 5, 7, 9, 11, 13, 15, 17, 19-21, 101, 123, 125, 127, 129, 131, 150, 1024, 1666, 1890, 1990
or
switchport trunk allowed vlan 1, 3, 5,
switchport trunk allowed vlan add 7, 9, 11
switchport trunk allowed vlan add 13, 15, 17, 19-21
switchport trunk allowed vlan add 101, 123, 125 1024
switchport trunk allowed vlan add 1666, 1890, 1990
switchport trunk allowed vlan add 127, 129, 131, 150
(or even 1 per line)
but will always show in the configuration as:
switchport trunk allow vlan [as many as can fit on a single displayable line with no truncate]
switchport trunk allowed vlan add [as many as can fit on a single line with no truncate]
and further statements as necessary.
E.g no matter what you write the following will happen:
The VLANS will display in numerical order, (e.g. those 127, 129, 131, 150 vlans in the example above will be shifted from the end to midway through)
truncation will always be applied if it is appropriate:
switchport trunk allowed vlan 1, 2, 3, 4, 5
will always show as:
switchport trunk allowed vlan 1-5
the line will never overflow the line buffer of the console.
(e.g huge list of VLANS is truncated to a vlan a,b,c, then followed by as many vlan add d,e,f lines until it takes to get to the end of the list.)
so in the example:
switchport trunk allowed vlan 31,55,100,113-115,171-172,208-209,212-216
adding vlans 1210-1214
may rightfully show as
switchport trunk allowed vlan 31,55,100,113-115,171-172,208-209,212-216
switchport trunk allowed vlan add 1210-1214
