If you change minecraft:generic.max_health using /attribute command, it doesn't update your current HP.
This is not a big problem for increasing health, since it will regenerate. However, if you lower your health, it stays the same until updating it via taking damage - it will then lower it to current max value, no matter how strong the hit was.
How to reproduce:
Regenerate to full 20 HP.
Use /scoreboard objectives add hp health and /scoreboard objectives setdisplay sidebar hp to see a scoreboard displaying true health.
Use /attribute @s minecraft:generic.max_health base set 10.
Notice how your HP stays the same - 20.
Take damage. Notice how HP changes to 10.
Environment
~ $ inxi -Fx
System:
Host: pecet Kernel: 5.6.0-14.1-liquorix-amd64 x86_64 bits: 64
compiler: gcc v: 9.3.0 Desktop: KDE Plasma 5.17.5
Distro: Debian GNU/Linux bullseye/sid
Machine:
Type: Desktop System: Dell product: OptiPlex 790 v: 01
serial: <superuser/root required>
Mobo: Dell model: 0J3C2F v: A00 serial: <superuser/root required>
BIOS: Dell v: A07 date: 09/10/2011
CPU:
Topology: Quad Core model: Intel Core i5-2400 bits: 64 type: MCP
arch: Sandy Bridge rev: 7 L2 cache: 6144 KiB
flags: avx lm nx pae sse sse2 sse3 sse4_1 sse4_2 ssse3 vmx bogomips: 24745
Speed: 3156 MHz min/max: 1600/3101 MHz Core speeds (MHz): 1: 3268 2: 2384
3: 2833 4: 3081
Graphics:
Device-1: NVIDIA GK106 [GeForce GTX 660] vendor: Micro-Star MSI
driver: nvidia v: 440.82 bus ID: 03:00.0
Display: x11 server: X.Org 1.20.8 driver: nvidia resolution: 1280x1024
OpenGL: renderer: GeForce GTX 660/PCIe/SSE2 v: 4.6.0 NVIDIA 440.82
direct render: Yes
Audio:
Device-1: Intel 6 Series/C200 Series Family High Definition Audio
vendor: Dell driver: snd_hda_intel v: kernel bus ID: 00:1b.0
Device-2: NVIDIA GK106 HDMI Audio vendor: Micro-Star MSI
driver: snd_hda_intel v: kernel bus ID: 03:00.1
Sound Server: ALSA v: k5.6.0-14.1-liquorix-amd64
Network:
Device-1: Intel 82579LM Gigabit Network vendor: Dell driver: e1000e
v: 3.2.6-k port: 4040 bus ID: 00:19.0
IF: eno1 state: up speed: 100 Mbps duplex: full mac: 18:03:73:b0:66:be
IF-ID-1: lxcbr0 state: down mac: 00:16:3e:00:00:00
Drives:
Local Storage: total: 1.02 TiB used: 728.01 GiB (69.8%)
ID-1: /dev/sda vendor: GOODRAM model: N/A size: 111.79 GiB
ID-2: /dev/sdb vendor: Seagate model: ST1000DM003-1SB10C size: 931.51 GiB
Partition:
ID-1: / size: 58.81 GiB used: 22.88 GiB (38.9%) fs: ext4 dev: /dev/sda3
ID-2: /home size: 901.04 GiB used: 702.91 GiB (78.0%) fs: ext4
dev: /dev/sdb1
Swap:
ID-1: swap-1 type: partition size: 15.09 GiB used: 2.21 GiB (14.7%)
dev: /dev/sdb5
Sensors:
System Temperatures: cpu: 65.0 C mobo: N/A gpu: nvidia temp: 54 C
Fan Speeds (RPM): N/A gpu: nvidia fan: 38%
Info:
Processes: 268 Uptime: 13h 15m Memory: 7.73 GiB used: 4.94 GiB (63.9%)
Init: systemd runlevel: 5 Compilers: gcc: 9.3.0 clang: 9.0.1-12 Shell: zsh
v: 5.8 inxi: 3.1.00
~ $ java --version
openjdk 15-ea 2020-09-15
OpenJDK Runtime Environment (build 15-ea+19-Debian-1)
OpenJDK 64-Bit Server VM (build 15-ea+19-Debian-1, mixed mode, sharing)
~ $
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