Bug?
Hardware - RB493AH
Software - RouterOS v.3.30 mipsbe

[admin@MikroTik] /ip accounting> export
# ....
/ip accounting
set account-local-traffic=yes enabled=yes threshold=8192
/ip accounting web-access
set accessible-via-web=yes address=0.0.0.0/0

tcpdump from web-client:
18:36:16.326763 IP 212.0.65.x.60012 > 212.0.69.y.80: S 2253033990:2253033990(0) win 65535 <mss 1460>
0x0000: 4500 002c 8c97 4000 4006 7fcf d400 4102 E..,..@.@.....A.
0x0010: d400 4562 ea6c 0050 864a 9206 0000 0000 ..Eb.l.P.J......
0x0020: 6002 ffff 2e84 0000 0204 05b4 `...........
18:36:16.328808 IP 212.0.69.y.80 > 212.0.65.x.60012: S 1197448033:1197448033(0) ack 2253033991 win 4392 <mss 1380>
0x0000: 4500 002c 0000 4000 3f06 0d67 d400 4562 E..,..@.?..g..Eb
0x0010: d400 4102 0050 ea6c 475f 9b61 864a 9207 ..A..P.lG_.a.J..
0x0020: 6012 1128 7309 0000 0204 0564 0000 `..(s......d..
18:36:16.328839 IP 212.0.65.x.60012 > 212.0.69.y.80: . ack 1 win 65535
0x0000: 4500 0028 8c98 4000 4006 7fd2 d400 4102 E..(..@.@.....A.
0x0010: d400 4562 ea6c 0050 864a 9207 475f 9b62 ..Eb.l.P.J..G_.b
0x0020: 5010 ffff 2e80 0000 P.......
18:36:16.328884 IP 212.0.65.x.60012 > 212.0.69.y.80: P 1:117(116) ack 1 win 65535
0x0000: 4500 009c 8c99 4000 4006 7f5d d400 4102 E.....@.@..]..A.
0x0010: d400 4562 ea6c 0050 864a 9207 475f 9b62 ..Eb.l.P.J..G_.b
0x0020: 5018 ffff 2ef4 0000 4745 5420 2f61 6363 P.......GET./acc
0x0030: 6f75 6e74 696e 672f 6970 2e63 6769 2048 ounting/ip.cgi.H
0x0040: 5454 502f 312e 300d 0a55 7365 722d 4167 TTP/1.0..User-Ag
0x0050: 656e en
18:36:16.331055 IP 212.0.69.y.80 > 212.0.65.x.60012: . ack 117 win 4392
0x0000: 4500 0028 4b11 4000 3f06 c259 d400 4562 E..(K.@.?..Y..Eb
0x0010: d400 4102 0050 ea6c 475f 9b62 864a 927b ..A..P.lG_.b.J.{
0x0020: 5010 1128 8a02 0000 0000 0000 0000 P..(..........
18:36:16.331061 IP 212.0.69.y.80 > 212.0.65.x.60012: R 1:1(0) ack 117 win 4392
0x0000: 4500 0028 4b12 4000 3f06 c258 d400 4562 E..(K.@.?..X..Eb
0x0010: d400 4102 0050 ea6c 475f 9b62 864a 927b ..A..P.lG_.b.J.{
0x0020: 5014 1128 89fe 0000 0000 0000 0000 P..(..........

Problem solved.

/ip service> print
Flags: X - disabled, I - invalid
# NAME PORT ADDRESS CERTIFICATE
0 telnet 23 ...
1 ftp 21 ...
2 www 80 212.0.65.x
3 ssh 22 ...
4 X www-ssl 443 0.0.0.0/0 none
5 X api 8728 0.0.0.0/0
6 winbox 8291 ...