Unexpected output of top when started by systemctl

I want to log the cpu and memory usage of a service which starts some time after startup (webappmgr.service). For this I have created another simple logging service which depends on it and shall be run after webappmgr starts called perftrace.service. This shall continuously write the output of top and awk to the file /tmp/webapp.top:

[Unit]
Description=Perftrace
After=webappmgr.service

[Service]
User=root
Type=oneshot
RemainAfterExit=yes
ExecStart=/bin/bash -c 'top -b -H -p $(pgrep webappmgr) | awk \'{ print strftime("%%H:%%M:%%S", systime())" " $0 }\' > /tmp/webapp.top &'

[Install]
WantedBy=webappmgr.service

When the system is started up and webappmgr is running, I start perftrace by systemctl start perftrace. In the file /tmp/webapp.top I see what I want to see:

04:26:27  2596 testuser  20   0 1887640  24092  17068 S  0.0  0.5   0:00.27 webappmgr
04:26:27  2622 testuser  20   0 1887640  24092  17068 S  0.0  0.5   0:00.06 JSEngine
04:26:27  2628 testuser  20   0 1887640  24092  17068 S  0.0  0.5   0:00.01 webappmgr
04:26:27  2629 testuser  20   0 1887640  24092  17068 S  0.0  0.5   0:00.00 webappmgr
04:26:27  2630 testuser  20   0 1887640  24092  17068 S  0.0  0.5   0:00.00 webappmgr
04:26:27  2631 testuser  20   0 1887640  24092  17068 S  0.0  0.5   0:00.00 webappmgr
04:26:27  2632 testuser  20   0 1887640  24092  17068 S  0.0  0.5   0:00.00 FileDownloa+

However, after enabling the perftrace.service and rebooting the system, so that it starts at the startup right after webappmgr, I see a completely different ouput in the file:

04:31:03 %Cpu(s): 33.1 us, 17.7 sy,  0.1 ni, 41.2 id,  1.2 wa,  4.3 hi,  2.5 si,  0.0 st
04:31:03 KiB Mem :  4667484 total,  1369968 free,  2187548 used,  1109968 buff/cache
04:31:03 KiB Swap:        0 total,        0 free,        0 used.  2373988 avail Mem
04:31:03
04:31:06   PID USER      PR  NI    VIRT    RES    SHR S %CPU %MEM     TIME+ COMMAND
04:31:06
04:31:06 top - 04:31:06 up 1 min,  1 user,  load average: 40.87, 12.93, 4.54
04:31:06 Threads:   0 total,   0 running,   0 sleeping,   0 stopped,   0 zombie
04:31:06 %Cpu(s): 11.2 us, 15.5 sy,  0.0 ni, 66.8 id,  0.8 wa,  3.5 hi,  2.1 si,  0.0 st
04:31:06 KiB Mem :  4667484 total,  1362788 free,  2194416 used,  1110280 buff/cache
04:31:06 KiB Swap:        0 total,        0 free,        0 used.  2367116 avail Mem
04:31:06
04:31:09   PID USER      PR  NI    VIRT    RES    SHR S %CPU %MEM     TIME+ COMMAND
04:31:09
04:31:09 top - 04:31:09 up 1 min,  1 user,  load average: 40.87, 12.93, 4.54
04:31:09 Threads:   0 total,   0 running,   0 sleeping,   0 stopped,   0 zombie
04:31:09 %Cpu(s): 11.9 us, 16.2 sy,  0.0 ni, 66.0 id,  0.0 wa,  3.6 hi,  2.2 si,  0.0 st
04:31:09 KiB Mem :  4667484 total,  1358060 free,  2199204 used,  1110220 buff/cache
04:31:09 KiB Swap:        0 total,        0 free,        0 used.  2362388 avail Mem
04:31:09

what shall I do to get similar output to when I started my perftrace service manually?

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